Abstract: Pneumatic seed drill has been the high-speed, wide-spread seeding in recent years, due to the small weight, high efficiency, low seed damage, especially suitable for the wheat, pasture, rice, and oil seed rape crops. Among them, the pneumatic set row system has been adopted for the quantitative seed supply, airflow conveying, and uniform distribution. Its distribution device is one of the most important working parts. Specifically, the seed flow in the distribution device is uniformly distributed into rows by the outlet discharge. The distribution device structure can dominate the pneumatic set row planter rows. The rows of the outlet number volume can also determine the pneumatic set row planter rows of seeds. This study aimed to investigate the pneumatic seed drill for the sowing rows of the adjustment. The distribution device in the flow field led to the deterioration of the discharge consistency of its rows. There was no smoothness in the seed discharge. An adjustable row distribution device was designed with a maximum of 20 rows. The overall structure and working principle were elaborated for the adjustable-row pneumatic centralized distribution. The parameter range was determined for the main operation of the seeder and the key structure in the adjustable-row distribution device. A systematic investigation was made on the force movement and pressure loss of the seeds in the contraction tube section. After that, the contraction angle, contraction section height, and backflow section length were the key influencing parameters on the gas-seed two-phase flow movement. According to the CFD-DEM coupled simulation, a one-factor test was conducted with the Box-Behnken response surface experimental design, in order to explore the impacts of the contraction angle, contraction section height, and reflux section length on the coefficient of variation in the consistency of discharge in each row and the seed retention rate in the reflux chamber. Simulation results show that the better performance was achieved at the contraction angle of 12°, where the height of the contraction section was 54 mm, and the length of the reflux section was 28 mm, the coefficient of variation in the consistency of the rows of discharge was 6.73%, and the retention rate of seeds in the reflux chamber was 4.30%. The pneumatic centralized seeding test bench was developed to conduct the seeding experiments. As such, there were both the adjustable row number distribution device under different combinations of the open/closed rows, and the conventional M-type distribution device. The results show that the row-to-row discharge consistency of the adjustable row number distribution device significantly outperformed the conventional M-type device (which adjusts seeding rows by blocking seed tubes), indicating higher consistency levels. There was a 5.70% consistency variation coefficient of the row displacement in the same application scenario as the simulation test, which was 1.03 percentage points different from the simulation. The consistency variation coefficient of the row displacement of the adjustable row number distribution device was better than that of the ordinary ones. The coefficient of variation of the row volume consistency was at a lower level with the increase in the number of consecutively closed seed guides. There was a greater difference in the row volume consistency between the ordinary and the row number-adjustable distribution device. The high-speed photography showed that there was no outstanding seed retention in the reflux chamber after the seed supply device stopped for 2 s. The adjustable row number distribution device improved the uniformity of the metering consistency across all rows in pneumatic centralized metering systems, compared with the conventional M-type ones, particularly with the adjusted number of seeding rows. The finding can provide technical support to the pneumatic set-row planter, in order to realize rapid adjustments on the number of rows for high-quality seeding.